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In a cylindrical beam, which strongly illuminated the
dust of the laboratory, was placed an ignited spirit-lamp.
Mingling with the flame, and round its rim, were seen
curious wreaths of darkness resembling an intensely-black
smoke. On lowering the flame below the beam the same
dark masses stormed upward. They were at times blacker
than the blackest smoke that I have ever seen issuing from
the funnel of a steamer; and their resemblance to smoke
was so perfect as to lead the most practised observer to
conclude that the apparently-pure flame of the alcohol-lamp
required but a beam of sufficient intensity to reveal its
clouds of liberated carbon.

But is the blackness smoke? This question presented
itself in a moment. A red-hot poker was placed under-
neath the beam, and from it the black wreaths also as-
cended. A large hydrogen-flame was next employed, and
it produced those whirling masses of darkness far more
cOpiously than either the spirit-flame or poker. Smoke was
therefore out of the question.

What, then, was the blackness? It was simply that of
stellar space; that is to say, blackness resulting from the
absence from the track of the beam of all matter competent
to scatter its light. When the flame was placed below the
beam, the floating matter was destroyed in Sim ; and the
air, freed from this matter, rose into the beam, jostled aside
the illuminated particles, and substituted for their light the
darkness due to its own perfect transparency. Nothing
could more forcibly illustrate the invisibility of the agent
which renders all things visible. The beam crossed, un-
seen, the black chasm formed by the transparent air, while
at both sides of the gap the thick-strewn particles shone
out like a luminous solid under the powerful illumina-

But here a rather perplexing difficulty meets us. It is
not necessary to burn the particles to produce a stream of

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